1. Field of the Invention
The present invention relates to an optical module used in optical communications, and more particularly to an optical module, which has high airtightness, and a constant plane of polarization of transmitted light.
2. Description of the Related Art
In the field of optical communications, which has developed phenomenally in recent years, optical module airtightness is seen as being important for the reliable transmission of a light signal. This results from the electrode of an optical semiconductor device arranged on the inside of an optical module deteriorating when, the inside thereof constitutes a high-temperature, high-humidity state, and from the optical properties of an optical semiconductor device deteriorating due to the condensing of moisture that penetrates to the inside, making it impossible to guarantee the life of an optical semiconductor device for more than 10 years.
However, an optical module has the role of using a lens to optically couple an inside optical semiconductor device and an outside optical fiber. To ensure the airtightness of an optical module as-is and maintain the optical system thereof, a, light transmitting-type window structure is employed in an optical semiconductor airtight-sealed housing.
As the window material of the housing (airtight-sealed housing) for an optical module, sapphire is often utilized due to its excellent translucence and high strength. Japanese Patent Application Laid-open No. H8-148594 discloses a basic structure and manufacturing method of a housing for an optical module, which uses sapphire in a light transmitting-type window. Regarding to the window structure of the housing thereof, Japanese Patent Application Laid-open No. H8-148594 discusses the relationship between the optical axis and the C axis of the sapphire. Here, a window structure is proposed, in which an optical axis, which refracts according to Snell""s law, is made coincident with the C axis of the window plate, so that occurrence of light birefringence, that is, rotation of the plane of polarization of light is prohibited.
In Japanese Patent Application Laid-open No. H11-54642, an optical module window structure, which uses borosilicate glass as the window plate, is proposed. Borosilicate glass is inexpensive, and its translucency surpasses even that of sapphire. Furthermore, borosilicate glass is an isotropic material, and does not cause the birefringence of light. However, the problem with borosilicate glass is that the plane of linear polarization of transmitted light deforms, namely the plane is not maintained as-is and the light changes into ellipsoidal polarization in some cases, due to the elastic strain resulting from heat stress. But as disclosed in Japanese Patent Laid-open No. 11-54642, it has become clear that the deformation of the plane of polarization of light can be minimized by applying a stress uniformly to the glass, at which time the polarized light extinction ratio described below, which serves as an index of the deformation of the plane of polarization of light, decreases to around xe2x88x9240 dB, and does not pose a problem from the standpoint of practical use.
Here, the deformation of the plane of polarization of light is generally expressed as a polarized light extinction ratio such as the following. In a crossed Nicol prism experimental system, when the polarizer of the light-emitting side was made to rotate 90 degrees relative to that of the light-receiving side, if the maximum light intensity in the light-receiving side is labeled Imax, and the minimum light intensity is labeled Imin, then the polarized light extinction ratio is defined as 10xc3x97log10 (Imin/Imax). Therefore, this indicates that the smaller the polarized light extinction ratio is, the smaller the deformation of the plane of polarization of light becomes.
In line with the recent advances in technology for increasing transmission speeds and high-density wavelength-multiplexing technology in optical communications, maintaining the constant plane of polarization of transmitted light and ensuring the uniformity of the wavelength thereof have become important tasks. To solve for the latter of these, it is desirable to form an external resonator structure, such as an optical fiber grating, via an optical fiber on the outside of an airtight-sealed housing, but maintaining the constant plane of polarization of light is required in this case as well. The level of plane of polarization maintenance required at this time is the above-mentioned polarized light extinction ratio of xe2x88x9230 dB, that is, a level at which Imax is 1000-times greater than Imin, making this a stringent condition incapable of being achieved with the above-mentioned conventional technology.
Furthermore, in the above-mentioned conventional technology, which uses sapphire as the window plate, a method such as the following had to be utilized to achieve the desired relationship between an optical axis and the C axis of sapphire. That is, it was necessary either to use a sapphire plate, which had been cut perpendicular to the C axis, and to make the C axis correspond to the optical axis by arranging the sapphire plate thereof perpendicular to the optical axis, or, when that was not the case, to go to the trouble of grinding and manufacturing a sapphire plate, having a specified angle relative to the C axis, and furthermore, aligning the C axis position of the sapphire plate thereof exactly with the optical axis. In the case of the former, reflected light directly returns to the incoming side, making it unsuitable for an optical module. In the case of the latter, exact alignment was difficult, and it was impossible to manufacture a window part by accurately fixing the angle.
Further, even when borosilicate glass is used as the window plate, borosilicate glass has the drawback of having weak strength. Therefore, a borosilicate glass window plate was not suitable for use under harsher conditions, and was an unsatisfactory material for practical use. In fact, the current situation is such that the use of borosilicate glass is still being shunned in fields in which ultra-high reliability is required for undersea cables.
Accordingly, an object of the present invention is to provide an optical module having a window structure, which is easy to manufacture, has mechanical strength, and has a small polarized light extinction ratio, holding the plane of linear polarization constant.
According to the present invention, there is provided an optical module comprising:
a housing; and
a junction having a light transmissive window structure which uses a sapphire plate,
wherein the following expressions (1)-(4) are established between an angle "psgr" as viewed from the optical axis, formed by the C axis of the sapphire and the plane of polarization of light, which is linear polarization, and an angle xcex8 formed by the C axis of the sapphire and the optical axis.
n=xcfx89xcex5/{square root over ((xcfx892cos2xcex8+xcex52sin 2xcex8))}xe2x80x83xe2x80x83(1)
xcex4=2xcfx80d(xcfx89xe2x88x92n)/xcexxe2x80x83xe2x80x83(2)
                              tan          ⁢                      xe2x80x83                    ⁢          β                =                                                            (                                                                            sin                      2                                        ⁢                    2                    ⁢                                          xe2x80x83                                        ⁢                    ψ                    ⁢                                          xe2x80x83                                        ⁢                                          sin                      2                                        ⁢                                          xe2x80x83                                        ⁢                    δ                                    +                  1                                )                                      -            1                                sin            ⁢                          xe2x80x83                        ⁢            2            ⁢                          xe2x80x83                        ⁢            ψ            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢            δ                                              (        3        )            xe2x80x83xe2x88x9230xe2x89xa7+20log(tanxcex2)xe2x80x83xe2x80x83(4)
where,
xcfx89: Principal refractive index of the sapphire
xcex5: Secondary refractive index of the sapphire
xcex: Wavelength of a transmitted light
d: Thickness of the sapphire
Furthermore, according to the present invention, there is provided an optical module according to the above-mentioned optical module, wherein, when the thickness d of said sapphire plate is 0.28 mm, and N is an integer, any one of the following expressions (5) through (7) is established between said angle "psgr" and said angle xcex8.
6 degreesxe2x89xa6xcex8xe2x89xa610 degrees and (90Nxe2x88x929) degreesxe2x89xa6"psgr"xe2x89xa6(90N+9) degreesxe2x80x83xe2x80x83(5)
2 degreesxe2x89xa6xcex8 less than 6 degrees and (90Nxe2x88x9232) degreesxe2x89xa6"psgr"xe2x89xa6(90N+32) degreesxe2x80x83xe2x80x83(6)
10 degrees less than xcex8xe2x89xa614 degrees and (90Nxe2x88x925) degreesxe2x89xa6"psgr"xe2x89xa6(90N+5) degreesxe2x80x83xe2x80x83(7)
First, the angle "psgr" formed by the C axis of the sapphire and the plane of polarization of light, and the angle xcex8 formed by the C axis of the sapphire and the optical axis will be explained using FIG. 2. In FIG. 2(A), the solid line arrow represents the optical axis, and the direction of the arrow thereof is the traveling direction of the light. The rectangle in the center of FIG. 2(A) is a cross-section of a sapphire plate, and the broken line arrow represents the C axis of the sapphire crystal. The sapphire plate is cut and ground so as to be perpendicular to the C axis. As shown in this figure, the angle formed by the optical axis and the C axis of the sapphire is xcex8. FIG. 2(B) is a figure, which views (A) from the incident light side. A circular sapphire plate is shown in this figure, and the optical axis is perpendicular to the surface of the paper. The solid line double arrow of FIG. 2(B) indicates the direction of polarization of incoming light. The broken line arrow of FIG. 2(B) indicates the projection toward the surface of the paper (surface which is perpendicular to the optical axis) of the C axis of the sapphire crystal, and the actual C axis is heading diagonally upwards from the paper surface by the amount of xcex8. As shown in FIG. 2(B), the angle formed by this projection of the C axis and the direction of polarization of light is "psgr".
In an optically uniaxial crystal like sapphire, if the C axis of the crystal is made correspond to the optical axis; the occurrence of birefringence is prohibited, and the plane of polarization of incoming light is maintained without rotation. However, as explained hereinabove, there are the affects of reflected return light, and the difficulties from the standpoint of manufacturing. On the other hand, the inventors, through experimentation, have formed that, if incoming light is linearly polarized, birefringence can be prevented even without making the optical axis of incident light and the C axis of the sapphire correspond to one another, by making the angle formed by the plane of polarization of incoming light and the C axis equal to 0, and setting both to the same plane. In addition, it was also found that to attain a polarized light extinction ratio of xe2x88x9230 dB, which is a sufficient condition required in the field of optical communications, in the end it is efficient to set the plane of polarization of light and the C axis to the same plane. However, setting the plane of polarization of light and sapphire C axis thereof to exactly the same plane is extremely difficult from the standpoint of manufacturing a housing.
Accordingly, the inventors, as a result of research, succeeded in achieving approximate expressions, which are capable of describing with high accuracy the polarized light extinction ratio of transmitted light when handling only linear polarization, by using the angle xcex8 formed by the optical axis of incident light and the C axis of sapphire; the angle "psgr" formed by the plane of polarization of incoming light and the C axis of sapphire; the principle index of refraction xcfx89 of the sapphire; the secondary refractive index xcex5 of the sapphire; the wavelength xcex of the transmitted light; and the thickness d of the sapphire plate. These are the expressions on the (1)-(3) and the right side of (4) described hereinabove. The graphs, of FIG. ""s 4-6, respectively, show the dependence of the polarized light extinction ratio on measured values and calculated values of xcex8 and "psgr" when d is changed. From these graphs, it is clear that the measured values and values calculated using the approximate expressions match up well.
Furthermore, as a result of prototype testing, the inventors discovered the optimum range that enables the realization of a desired polarized light extinction ratio. The inequality of the above-mentioned (4), and the expressions of (5) through (7) when d is 0.28 mm is the optimum range thereof. That is, even if the optical axis of incoming light and the C axis of the sapphire do not correspond, by setting the angle formed by the plane of polarization of the light and the C axis thereof to be small, the condition of a polarized light extinction ratio of xe2x88x9230 dB or less can be satisfied. Conversely, even if manufacturing is made easier by making the angle of the plane of polarization of incoming light and the C axis of the sapphire large, by setting the angle formed by the optical axis of incoming light and the C axis to be small, the condition of xe2x88x9230 dB or less can be satisfied. When (4) is realized, it is desirable for the thickness d of the sapphire plate to be less than 0.3 mm. In the embodiments, which will be explained below, a sapphire plate 0.28 mm thick was used.
Further, by using a semiconductor laser, and connecting a polarization-maintaining (PANDA) fiber to an optical module of the present invention, it is possible to transmit light for which the plane of polarization of the linear polarized light is more accurately maintained at a high level as-is. In this case, a fiber optic amplifier can be utilized efficiently by polarization synthesizing a plurality of excitation lights necessary for a fiber optic amplifier. Therefore, the efficient amplification of a transmitted light signal is possible. Further, the structure of an isolator, which is used together with the optical module of the present invention in optical communications, can be simplified, making possible lower costs.
Further, in a modulator, which uses an optical module of the present invention, and which is manufactured using LN (LiNbO3), which is an anisotropic optical material, since the polarized light extinction ratio of the window is small, it is possible to suppress the birefringence generated inside the LN modulator, enabling the realization of a light signal with a good S/N (signal-to-noise) ratio.
Further, when a semiconductor laser is mounted inside the optical module of the present invention and used, the light loss in the isolator connected outside the window can be held in check.
Furthermore, the optical module of the present invention can comprise a quarter wavelength plate (xcex/4 plate). In the past, in a semiconductor light amplifier, the problem was that polarization dependence occurred in the amplification characteristics. However, if a semiconductor light amplifier, which uses the optical module of the present invention, is used after setting the incoming light to the optical module to linear polarization via the xcex/4 plate, there is no polarization dependence, and amplification characteristics can be improved. After achieving linear polarization by inserting a xcex/4 plate, if the plane of polarization is aligned relative to the C axis of the sapphire, and the xcex/4 plate is also YAG welded, the amplification characteristics of a semiconductor light amplifier can be improved even further. Therefore, the insertion loss of a wavelength converting device, or a highspeed operation-capable optical-optical switching device, which is utilized in the semiconductor light amplifier thereof, can be reduced, enabling the realization of an optical signal with a good S/N ratio.
And furthermore, by providing a device which has reflection mechanism for selectively reflecting specified wavelengths, for example, an optical fiber grating, on the outside of the optical module of the present invention, it is possible to make this reflection device resonate together with an optical device on the inside of the optical module. Since the optical, signal oscillation mode disturbance generated by the birefringence of the window can be suppressed at this time, and furthermore, optical signal loss can also be reduced, the optical strength of the signal increases.
If N is set to 0 or an even number in the above-mentioned relational expressions (5)-(7), when light is incident on the sapphire plate, absorption ceases to occur due to the polarization of the sapphire. Thus, the transmittance of the sapphire plate becomes higher, and an optical signal can be transmitted without loss.